Heat resistant resins such as polyimide, polybenzoxazole, polybenzothiazole, and polybenzimidazole have been used as materials for various electronic devices such as semiconductors and display devices because of their good electrical insulation properties, heat resistance, and mechanical properties. Recently, production of shock resistant, flexible image display devices have become possible by applying heat resistant resin film to the substrates of image display devices such as organic EL displays, electronic paper, and color filters.
To apply a heat resistant resin as material for a display substrate, a common method is to spread a solution containing a heat resistant resin or a precursor thereof (hereinafter referred to as varnish) over a support, followed by firing the coating to form a film. An effort that uses N-methylpyrrolidone, which is high in ability to dissolve resins, as a solvent for resin compositions for display substrates has been reported (Patent document 1).
If a heat resistant resin film is used as display substrate, there will occur a problem associated with the poor barrier properties of the resin film, leading to easy infiltration of moisture and oxygen into the display internal. In organic EL displays, in particular, the light emitting devices can be deactivated easily due to infiltration of moisture, so the display substrates require good barrier properties. As a solution to solve this problem, a display substrate with good barrier properties having a plurality of layers of resin film and inorganic film has been reported (Patent document 2).